The cross-metathesis of two reactant olefins, where each reactant olefin comprises at least one unsaturation site, to produce new olefins which are different from the reactant olefins is of significant commercial importance. The cross-metathesis reaction is usually catalyzed by one or more catalytic metals, usually one or more transition metals.
One such commercially significant application is the cross-metathesis of ethylene and internal olefins to produce alpha-olefins, which is generally referred to as ethenolysis. In particular, the cross-metathesis of ethylene and an internal olefin to produce linear alpha-olefins (LAOS) is of particular commercial significance. LAOs are useful as monomers or comonomers in certain (co)polymers (polyalphaolefins or PAOs) and/or as intermediates in the production of epoxides, amines, oxo alcohols, synthetic lubricants, synthetic fatty acids and alkylated aromatics. Olefins Conversion Technology™, based upon the Phillips Triolefin Process, is an example of an ethenolysis reaction converting ethylene and 2-butene into propylene. These processes use heterogeneous catalysts, such as tungsten and rhenium oxides, which have not proven effective for internal olefins containing functional groups such as cis-methyl oleate, a fatty acid methyl ester.
Methods for the production of polyalpha-olefins are typically multi-step processes that often create unwanted by-products and waste of reactants and energy. Full range linear alpha-olefins plants are petroleum-based, are inefficient, and result in mixtures of oligomerization products that typically yield Schulz-Flory distributions producing large quantities of undesirable materials. In recent years there have been new technologies implemented to produce “on purpose” linear alpha-olefins such 1-hexene and 1-octene through chromium-based selective ethylene trimerization or tetramerization catalysts. Alternatively, 1-octene has been produced via the telomerization of butadiene and methanol. Similar strategies are not currently available for the production of 1-decene.
1-decene is a co-product typically produced in the cross-metathesis of ethylene and methyl oleate. Alkyl oleates are fatty acid esters that can be major components in biodiesel produced by the transesterification of alcohol and vegetable oils or animal fats. Vegetable oils containing at least one site of unsaturation include canola, soybean, palm, peanut, mustard, sunflower, tung, tall, perilla, grapeseed, rapeseed, linseed, safflower, pumpkin corn and many other oils extracted from plant seeds. Alkyl erucates similarly are fatty acid esters that can be major components in biodiesel. Useful biodiesel compositions are those which typically have high concentrations of oleate and erucate esters. These fatty acid esters preferably have one site of unsaturation such that cross-metathesis with ethylene yields 1-decene as a co-product.
Biodiesel is a fuel prepared from renewable sources, such as plant oils or animal fats. To produce biodiesel, triacylglycerides (“TAG”), the major compound in plant oils and animal fats, are converted to fatty acid alkyl esters (“FAAE,” i.e., biodiesel) and glycerol via reaction with an alcohol in the presence of a base, acid, or enzyme catalyst. Biodiesel fuel can be used in diesel engines, either alone or in a blend with petroleum-based diesel, or can be further modified to produce other chemical products.
Cross-metathesis catalysts reported thus far for the ethenolysis of methyl oleate are typically ruthenium-based catalysts bearing phosphine or carbene ligands. Dow researchers in 2004 achieved catalysts turnovers of approximately 15,000 using the 1st generation Grubb's catalyst, bis(tricyclohexylphosphine)benzylidene ruthenium(IV) dichloride, (Organometallics 2004, 23, p. 2027). Researchers at Materia, Inc. have reported turnover numbers up to 35,000 using a ruthenium catalyst containing a cyclic alkyl amino carbene ligand, (WO 2008/010961). These turnovers were obtained with a catalyst reportedly too expensive for industrial consideration due to high costs associated with the catalysts being derived from a low yielding synthesis (See, Final Technical Report entitled “Platform Chemicals from an Oilseed Biorefinery” grant number DE-FG36-04GO14016 awarded by the Department of Energy). Additionally, the introduction of chelating isopropoxybenzylidene ligands has led to ruthenium catalysts with improved activities for metathesis reactions (J. Am. Chem. Soc. 1999, 121, p. 791). However, these ruthenium alkylidene catalysts are usually prepared by the reaction of ruthenium species with diazo compounds. The concerns associated with industrial scale reactions comprising diazo compounds have led to increased efforts to prepare ruthenium alkylidenes via alternate synthetic routes, such as using terminal alkynes or propargyl alcohols.
The synthesis of RuCl2(PCy3)2(3-phenylindenylene) has proven useful in providing an easy route to ruthenium alkylidenes which avoids costly diazo preparations (Platinum Metals Rev. 2005, 49, p. 33). Also, Furstner et al., J. Org. Chem., 2000, 65, pp. 2204-2207, have prepared (N,N′-bis(mesityl)imidazol-2-ylidene)RuCl2(3-phenylindenylene). However, these types of complexes have not proven effective in ethenolysis reactions.
Unsymmetrical N-heterocyclic carbene ligands have been prepared by Blechert and coworkers and complexed to ruthenium alkylidenes to form active metathesis catalysts (Organometallics 2006, 25, pp. 25-28). It was hypothesized that these complexes would give improved activity to that of the symmetrical analogs previously prepared by Grubbs and coworkers (Org. Lett. 1999, 1, pp. 953-956). These complexes were tested for catalytic activity in ring closing and cross-metathesis reactions. However, the catalysts were reported to be similar in activity to the symmetrical analogs, namely the Grubbs catalyst, 2nd generation (1,3-bis-(2,4,6-trimethylphenyl)-2-(imidazolidinylidene) (dichlorophenylmethylene)(tricyclohexylphosphine)ruthenium) and the expected improved activities were not observed.
In order to obtain an economically viable process for 1-decene production via the cross-metathesis of ethylene and biodiesel (derived from animal or vegetable oils), higher activity catalysts must be discovered. Thus there is a need for higher activity processes that produce desired products and co-products in commercially desirable ratios.
There remains a need for catalysts which demonstrate high activity and selectivity in ethenolysis which are capable of being synthesized by both mild and affordable synthetic routes. The instant invention's metathesis catalyst compounds provide both a mild and commercially economical and an “atom-economical” route to desirable olefins, in particular alpha-olefins, which in turn may be useful in the preparation of PAOs. More particularly, the instant invention's metathesis catalyst compounds demonstrate improved activity and selectivity towards ethenolysis products in ethylene cross-metathesis reactions.
The inventors have found that symmetrically substituted N-heterocyclic carbene ligands linked to ruthenium alkylidenes, though known to be cross-metathesis catalysts, tend to have low activity in the ethenolysis of methyl oleate. Surprisingly, an asymmetrically substituted N-heterocyclic carbene ligand linked to a ruthenium alkylidene yielded a catalyst that was more active than the symmetrical analog and very selective towards the ethenolysis of methyl oleate yielding 1-decene and methyl-9-decenoate.
Other references of interest include: U.S. Pat. Nos. 7,119,216; 7,205,424; US 2007/0043180; WO 2006/138166; WO 2008/010961; US 2007/0043180; U.S. Pat. No. 7,268,242; WO 2008/125568; WO 2008/046106; WO 2008/095785; WO 2008/140468; U.S. Pat. No. 7,312,331; and WO 2008/010961.
Other references of interest also include: a) “Synthesis and Reactivity of Olefin Metathesis Catalysts Bearing Cyclic (Alkyl)(Amino)Carbenes” Anderson et al., Angew. Chem. Int. Ed. 2007, 46, pp. 7262-7265; b) “Intramolecular ‘Hydroiminiumation’ of Alkenes: Applications to the Synthesis of Conjugate Acids of Cyclic Alkyl Amino Carbenes (CAACs)” Jazzar et al., Angew. Chem. Int. Ed. 2007, 46, pp. 2899-2902; c) “Kinetic Selectivity of Olefin Metathesis Catalysts Bearing Cyclic (Alkyl)(Amino)Carbenes” Anderson et al., Organometallics, 2008, 27, pp. 563-566; d) “A New Synthetic Method for the Preparation of Protonated-NHCs and Related Compounds” Jazzar et al., J. Organometallic Chemistry 691, 2006, pp. 3201-3205; e) “A Rigid Cyclic (Alkyl)(Amino)carbene Ligand Leads to Isolation of Low-Coordinate Transition Metal Complexes” Lavallo et al., Angew. Chem. Int. Ed., 2005, 44, pp. 7236-7239; f) “Stable Cyclic (Alkyl)(Amino)carbenes as Rigid or Flexible, Bulky Electron-Rich Ligands for Transition Metal Catalysts: A Quaternary Carbon Atom Makes the Difference” Angew. Chem. Int. Ed., 2007, 44, pp. 5705-5709; g) “Synthesis and Activity of a New Generation of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with 1,3-Dimesityl-4,5-dihydroimidazol-2-ylidene Ligands” Org. Letters, 1999, 1, pp. 953-956.